In certain lamps of the above type, it is important that the light-generating means be precisely located in a predetermined position within the envelope of the lamp. For example, in a metal-halide discharge lamp, light is generated by an arc developed between spaced electrode tips, and these tips must be precisely located with respect to each other and the surrounding envelope in order to achieve proper envelope, or arc-tube, temperature distribution and desired optical properties, life, and lumen output. The electrode tips are located by supporting them on leads which extend through openings in the envelope, and seals are formed between the envelope and the leads in order to support the leads on the envelope and to prevent any leakage in this region. The seals can be either of the pinch-seal type or the shrink-seal type.
One obstacle to achieving precise positioning of the electrode tips is the difficulty of making the seals without introducing such distortion of the sealing regions of the envelope that displaces the leads and electrode tips from the precise positions desired. We are concerned with providing a seal and support structure for the leads that lends itself to achieving precise positioning of the light-generating means, for example, the electrode tips in the case of a discharge lamp.
One technique that has been used for achieving the desired positioning of the electrode tips has been to provide a metal coil about the supporting lead and to seal the outer portion of this coil to the surrounding leg of the quartz envelope. Such a coil serves to restrict movement of the supporting lead and the electrode tip mounted thereon, thereby decreasing electrode runout, i.e., displacement of the electrode tip from its predetermined desired position. The coil also serves to keep the quartz from wetting the refractory-metal supporting lead and thus eliminates shaling (i.e., cracking of the quartz in the immediate region of any refractory-metal part bonded thereto) as a result of unequal thermal expansion of the quartz and the refractory metal. A coil of this general type is disclosed and claimed in U.S. Pat. No. 4,942,331--Bergman, assigned to the assignee of the present invention.
A significant disadvantage of using a coil in this location is that the coil leaves a large cavity at the back of the arc chamber of the lamp. During lamp operation, the cavity tends to operate at a much cooler temperature than the rest of the arc chamber, and, as a result, certain metallic and metal halide components of the arc-chamber fill condense there, causing inferior arc-tube performance.
Another disadvantage of using a coil in this location is that during starting the arc may attach to the coil end exposed to the interior of the arc chamber. This can overheat the quartz in the adjacent area and cause early lamp failure.
We are concerned with providing support and sealing structure which limits electrode runout, substantially eliminates shaling, and causes the region of the lamp at the back of the arc chamber to operate at an acceptable temperature.
We are also concerned with providing a repeatable end chamber shape and eliminating the use of small conducting wires which can provide an unintentional arc spot.